Each DC machine can act as a generator or a motor. Hence, thisclassification is valid for both:DC generatorsandDC motors. DC machines are usuallyclassified on the basis of their field excitation method

1)Separately excited DC machines:In separately excited dc machines, the field winding is supplied from a separate power source. That means the field winding is electrically separated from the armature circuit.Separately excited DC generatorsare not commonly used because they are relatively expensive due to the requirement of an additional power source or circuitry. They are used in laboratories for research work, for accuratespeed control of DC motorswith Ward-Leonard system and in few other applications where self-excited DC generators are unsatisfactory. In this type, the stator field flux may also be provided with the help of permanent magnets (such as inpermanent magnet DC motors). PMDC (permanant magnet DC) motors are popularly used in small toys, e.g. a toy car.

2)Self-excited DC machines:In this type, field winding andarmature windingare interconnected in various ways to achieve a wide range of performance characteristics (for example, field winding in series or parallel with the armature winding).In aself-excited type of DC generator, the field winding is energized by the current produced by themselves. A small amount of flux is always present in the poles due to the residual magnetism. So, initially, current induces in the armature conductors of a dc generator only due to the residual magnetism. The field flux gradually increases as the induced current starts flowing through the field winding.